In recent years, an device has become popular that handles image information digitally and for the purpose of highly efficiently transmitting and accumulating the information, compresses and encodes an image by employing an encoding method that compresses the image through the motion compensation and orthogonal transform such as discrete cosine transform by using the redundancy unique to the image information. This encoding method includes, for example, MPEG (Moving Picture Experts Group).
In particular, MPEG2 (ISO/IEC 13818-2) is defined as the versatile image encoding method, and is the standard covering both the interlaced scanning image and sequential scanning image and moreover the standard-resolution image and high-definition image. For example, currently MPEG2 is widely used in the applications for the professionals and consumers. By the use of the MPEG2 compression method, in the case of the interlaced scanning image with the standard resolution having 720×480 pixels, the code amount (bit rate) of 4 to 8 Mbps is allocated. By the use of the MPEG2 compression method, in the case of the interlaced scanning image with the high resolution having 1920×1088 pixels, the code amount (bit rate) of 18 to 22 Mbps is allocated. This enables the high compression rate and excellent image quality.
MPEG2 is mainly intended for the high-definition image encoding that is suitable for the broadcasting but does not deal with the lower code amount (bit rate) than MPEG1, i.e., with the encoding method with a higher compression rate. The encoding method as above is likely to be needed more as the portable terminals spread, and accordingly the MPEG4 encoding method has been standardized. In regard to the image encoding method, the specification was approved in Dec., 1998 as the international standard with the name of ISO/IEC 14496-2.
Moreover, in recent years, the standard called H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)) has been set for the purpose of encoding the image for the teleconference. It has been known that H.26L achieves higher encoding efficiency though H.26 requires more calculations in encoding and decoding than the conventional encoding methods such as MPEG2 and MPEG4. Moreover, as one of activities of MPEG4, based on this H.26L, the standardization that achieves higher encoding efficiency is performed as Joint Model of Enhanced-Compression Video Coding in which the function that is not supported in H.26L has been introduced.
As for the schedule of the standardization, the international standard was set with the name of H.264 and MPEG-4 part 10 (Advanced Video Coding, hereinafter AVC) in Mar., 2003.
In addition, as the extension of H.264/AVC, the standardization of FRExt (Fidelity Range Extension) including the quantization matrix or 8×8 DCT defined in MPEG-2 and the encoding tool necessary for the work, such as RGB, 4:2:2, and 4:4:4 was completed in Feb., 2005. In this manner, the encoding method capable of expressing even the film noises included in a film based on H.264/AVC is achieved and used in the wide application including Blu-Ray Disc (trademark).
In recent years, however, there has been an increasing desire for the encoding with a higher compression rate: compressing the image with approximately 4000×2000 pixels corresponding four times that of the high-vision image; or distributing the high-vision image in the environment with the limited transmission capacity such as on the Internet. This induces the further examination on the improvement of the encoding efficiency in VCEG under ITU-T.
In view of this, for the purpose of improving the encoding efficiency over AVC, JCTVC (Joint Collaboration Team-Video Coding) as the ITU-T and ISO/IEC joint standardization group has advanced the standardization of the encoding method called HEVC (High Efficiency Video Coding). As for the HEVC specification, Committee draft corresponding to the first draft was issued in February, 2012 (for example, see Non-Patent Document 1).
Incidentally, the conventional image encoding method such as the MPEG-2 or AVC has the scalability (scalability) function of encoding the image by dividing the image into a plurality of layers.
In other words, the image compression information of just a base layer (base layer) is transmitted to a terminal with low process capacity, such as a cellular phone, so that a moving image with low spatial temporal resolution or low image quality is reproduced; on the other hand, in addition to the information of the base layer, the image compression information of an enhancement layer (enhancement layer) is transmitted to a terminal with high process capacity, such as a TV or a personal computer, so that a moving image with high spatial temporal resolution or high image quality is reproduced. Thus, the image compression information depending on the capacity of the terminal or the network can be transmitted from a server without the transcoding process.
By the way, in the scalable encoding, performing the prediction process between the layers for all the pictures leads to the increase in calculation amount.
In view of this, specifying on/off (on/off) of the prediction process between the layers for every picture (picture) in the NAL unit (NAL_Unit) has been suggested (for example, see Non-Patent Document 2).